Natal Counselling
PRENATAL SCREENING TESTS FOR DOWN SYNDROME AND OTHER FETAL CONDITIONS
The aim of prenatal screening tests is to determine if the fetus has an increased risk of a chromosomal abnormality (such as Down syndrome) and to check for a physical birth defect. The tests are offered at 11 to 14 weeks, 15 to 17 weeks, and 18 to 20 weeks of pregnancy.
The results of these tests provide detailed information about risk and depend on accurate knowledge of the duration of the pregnancy (gestation).
Down syndrome:
Down syndrome is also known as Trisomy 21. If the fetus is affected, cells in the placenta and the fetus have an extra copy of chromosome 21. Intellectual disability is a feature of Down syndrome. Physical malformation, including hearing problems, visual problems and heart defects, may also occur. Down syndrome can occur in a fetus even if both parents have no family history of it.
Physical birth defects:
Ultrasound is used to detect physical defects. The most common physical birth defects seen on ultrasound are heart malformations, kidney abnormalities and neural tube defects (NTD). A NTD may be an open defect of the spinal cord (spina bifida) or the absence of the fetal skull and brain (anencephaly). If present, a NTD develops very early in fetal life.
Other conditions:
There are specific tests for many other conditions. Tests are conducted only when a parent is known to have a particular risk factor, such as a family history of an inherited condition.
The decision whether to have a prenatal screening test: A woman and her partner should have counselling before undergoing the tests. They should discuss their situation with their doctor, geneticist or genetic counsellor to gain a detailed understanding of the screening test and other related tests. Some women may find that the decision whether to have these tests is difficult. Do not make a decision in a rush but only when you are satisfied with the information that you have received and when you believe you have been well informed.
Consider the risks of having a child with a birth defect and the implications on your family's life. Consider how you would respond if an increased risk is found. If you have difficulty making a decision, you may want to seek a second opinion or consult a geneticist or genetic counsellor to discuss your concerns.
RISKS
AGE: The risk of having a baby with Down syndrome increases according to the woman's age, as shown in the table (left). The risk of some other rare chromosomal abnormalities also increases with age, for example, Trisomy 18, in which an extra copy of chromosome 18 occurs.
NEURAL TUBE DEFECT (NTD): The risk of having a baby with a NTD is about one in 1,000, provided the parents have no family history of a NTD or other predisposing factors (such as medications used to treat epilepsy).
SCREENING TESTS Ultrasound in the first trimester.
The benefits of having an ultrasound scan in the first trimester are:
- Accurate dating of the pregnancy.
- Detection of a pregnancy that is not progressing well.
- Early diagnosis of twins or other multiple pregnancies.
- Diagnosis of some fetal abnormalities.
Nuchal translucency: From 11 weeks . to 13 weeks and six days of pregnancy, an ultrasound scan can measure the thick-ness of fluid-filled tissue at the back of nuchal translucency" (NT). The thicker the NT, the greater the risk of Down syndrome.
Combined Test for Down syndrome : The Combined Test is the use of both ultrasound (for measurement of NT) and a blood test called first-trimester maternal serum screening (MSS).
The MSS blood test can be taken before the ultrasound or at the time of the ultra-sound, depending on the available testing facilities. The earliest time for MSS is 10 weeks gestation and the earliest time for the ultrasound is 11 weeks.
When both NT ultrasound and first-trimester MSS are performed, the assessment of risk for Down syndrome is more accurate than using either test alone. MSS estimates the levels of two substances: free 13-hCG (beta human chorionic gonadotrophin) and PAPP-A (pregnancy associated plasma protein A). Combining these results with the NT measurement and the woman's age provides an estimate of risk for Down syndrome.
If an increased risk is reported on a screening report, the woman will be offered a diagnostic test (chorionic villus sampling or amniocentesis). In most cases of increased risk, the fetus does not have Down syndrome. This is called a "false positive". Similarly, it is possible to have a "false negative" result where the fetus does have the condition but the test result indicates a low risk.
The Combined Test can identify up to nine pregnancies in every 10 that have a fetus with Down syndrome.
Second-trimester MSS
This test is best performed between 15 and 17 weeks of gestation. It screens for Down syndrome, Trisomy 18 and NTDs.
The woman's blood is tested for substances produced by the fetus or placenta, including alpha-fetoprotein (AFP), human chorionic gonadotrophin (hCG) and unconjugated oestriol.
These results plus the woman's age and gestation of the pregnancy are calculated together to estimate the risk of Down syndrome. MSS can identify about seven out of 10 pregnancies that have a fetus with Down syndrome. If the risk is increased, an amniocentesis is offered.
Ultrasound scan at 18 to 20 weeks Although ultrasound can be performed at any time during pregnancy, the' usual practice is to offer women an ultrasound scan at 18 to 20 weeks of gestation. The operator assesses the following:
- Size of the fetus compared with key dates (last menstrual period or early ultrasound results).
- Development of the fetus.
- Location of the placenta.
- Amount of amniotic fluid.
- Well-being of the fetus.
The accuracy of diagnosis depends on many factors, including the position and size of the fetus, the weight of the woman, operator experience, and the quality of the ultrasound machine.
Fetal abnormalities that ate diagnosed on ultrasound scan may need experts for interpretation of the problem and its consequences.
If there is concern about the findings, the doctor (in consultation with you and your partner and other specialists) may offer amniocentesis.
Amniocentesis can rule out or confirm the presence of chromosomal abnormalities and, when indicated, some genetic disorders.
Placement of ultrasound transducer
The ultrasound scan is usually performed with an abdominal ultrasound transducer. Sometimes, a woman may need a vaginal ultrasound to obtain accurate images.
RESULTS AND OPTIONS If the results of prenatal screening indicate an increased risk of the fetus having a chromosomal abnormality, you will be offered a prenatal diagnostic test. The College's patient education article titled "Amniocentesis and Chorionic Villus Sampling (CVS) — a guide on prenatal diagnostic procedures" may be helpful
Many women are clear about- what they want to do. However, some,women are very uncertain. Further counselling with their doctor, geneticist or a genetic counsellor can be helpful in many cases. The need for counselling support varies from woman to woman.
Limitations of ultrasound and MSS:
Screening tests are not perfect and cannot identify all fetuses with a chromosomal abnormality or other problem. A low-risk result does not mean that the fetus has no risk of a disorder being present. Similarly, an increased-risk result does not mean that the fetus is certain to have a disorder.
For example, some fetuses in the low-risk group will have Down syndrome, while some fetuses in the high-risk group will NOT have Down syndrome. However, most Down syndrome fetuses are in the high-risk group.
The level of risk : Although a screening test may indicate an increased risk of an abnormality, most women are still likely to have a normal baby. This is best explained in terms of "level of risk". Your level of risk is best discussed with, and explained by, your doctor, ultrasound specialist, a genetic counsellor, or midwife.
YOUR MEDICAL HISTORY
Your doctor needs to know your complete family and personal medical history. This includes any history of chromosomal disorders, neural tube defects or other fetal abnormalities. Give your doctor a list of ALL medicines and vitamins (including folic acid) you are taking or took around the time of conception.
Include medicines prescribed by your family doctor and those bought "over the counter" without prescription. This information is confidential.
COSTS OF SCREENING TESTS
Estimates of cost will be available from your doctor and the provider of the service (a pathology laboratory or ultrasound department). Ask about costs before your test rather than afterwards.
TALK TO YOUR DOCTOR
The aim of this article is I to provide you with general information. It is not a substitute for advice from your doctor or genetic counselor and does not contain all known facts about prenatal screening tests. If you are not sure about the benefits, disadvantages or limitations of these tests, or terms used in this article, ask your doctor. Read this article carefully. Write down questions you want to ask. Your doctor will be pleased to answer them, Seek the opinion of another doctor if you are uncertain about the advice you are given. Use this article only in consultation with your doctor or a genetic counselor.